In an ultrasound apparatus, presence of an artifact (virtual image) causes a reduction in resolution ability, which is known to be a serious problem to be solved. One of the causes of artifact generation is a side lobe (second main pole or subpole). The side lobe is an ultrasound wave radiated in another direction than a transmission direction of a main lobe of the ultrasound wave that has been radiated from an ultrasound probe. Types of side lobes include a grating lobe.
In the case of an ultrasound probe which linearly includes probes (linear probe), a direction of side lobe generation is determined by spacing between devices. For example, when an array of probes is designed based on a device pitch (space between devices) having a length equal to a wavelength of an ultrasound wave to be generated, a side lobe is formed in a direction tilted by 90° with respect to the transmission direction of the main lobe. In this case, when the main lobe is transmitted and received on a perpendicular plane of the linear probe, generation of artifacts can be reduced because of the forming direction of the side lobe tilted by 90° with respect to the transmission direction of the main lobe.
Similarly, even in the case of probes other than the linear probe, setting device pitches approximately equal to the above-mentioned wavelength enables a reduction of artifacts.
Generally, in designing of ultrasound probes, resolution abilities of a depth direction (transmission direction of ultrasound) and a direction perpendicular to the depth direction take priority, because of the importance in improving of resolution of a displayed image.
In designing, in view of productivity of ultrasound probes and sizes of transmission/reception circuits, it is difficult to shorten device pitches to approximately equal to the wavelength. Therefore, in such an ultrasound probe, a side lobe is generated in a direction of 90° or less with respect to the transmission direction of the main lobe.
Japanese Patent Application Laid-Open No. 2000-229080 discusses a method for suppressing artifacts generated by side lobes. Referring to FIG. 12, Japanese Patent Application Laid-Open No. 2000-229080 is described as follows.
Multiple oscillators that receive ultrasound are divided into two groups of A and B. Each oscillator transmits an ultrasound wave to receive a reflection wave from a test object. Signals received by the oscillators of the groups A and B are sent to reception circuits 400A and 400B, respectively. The reception circuits 400A and 400B carry out phase rectifying addition for the signals. Then, an adder 410 adds together the signals which have been subjected to the phase rectifying addition.
A phase reverser 420 reverses a phase of the signal received by the oscillators of the group A. An adder 411 adds together the phase-reversed signal and the signal received by the oscillators of the group B.
An absolute value calculator 430 calculates an absolute value of the signal added together by the adder 410 (detection processing) to output the absolute value as a whole received signal. An absolute value calculator 431 calculates an absolute value of the signal added together by the adder 411 (detection processing) to output the absolute value as a signal of an ultrasound wave from a transmission direction of a side lobe. Adding together the phase-reversed signal of one signal from the phase reverser 420 and the other signal enables extraction of signals different between the groups A and B. The signal corresponds to the signal of the ultrasound wave from the transmission direction of the side lobe.
When a subtracter 440 subtracts the signal of the ultrasound wave reflected from the direction of the side lobe from the whole received signal, a signal of an ultrasound wave from a transmission direction of a main lobe is left. Thus, an image having no artifacts can be displayed.
However, the unit for subtracting the side lobe discussed in Japanese Patent Application Laid-Open No. 2000-229080 is effective only when the inside of the test object is symmetrical with respect to the direction of the ultrasound transmitted/received by the oscillators of the groups A and B.
Generally, there is a sound-velocity distribution where the inside of the test object is asymmetrical with respect to a transmission/reception direction of an ultrasound wave. Thus, for signals obtained when refracted ultrasound waves are received by the oscillators, phases are different (asymmetrical) between the signals received by the groups A and B. As a result, even when a phase of the signal of one group is reversed to be added, an output signal cannot always be regarded as a signal corresponding to a side lobe.